Variable Height Whirling Weir-VHW Weir: Design and Hydraulic Performance

Water regulation and distribution structures are the main components of any irrigation network. If these structures fail, it has a direct impact on network performance and water loss. The type and shape of the regulation structure can be effective for better performance of the distribution structure. Studies show that one of the problems in irrigation networks is due to the mechanism of existing structures. At present, regulation structures with various shapes are used in the world. Rubicon Water has been working in Australia since 1995 to develop, build and install water regulation and distribution structures. The company's automated regulation structure, called FlumeGate, has been installed in different countries such as Australia, India, China and the United States. In the present study, the variable height whirling -VHW weir was introduced, designed, and constructed inspired by FlumeGate. The shape, mechanism and installation of this weir are relatively simple. The energy required to change its position is less than other gates. This is an overshot structure that has a better performance in the face of floating objects. Placing the weir crest at different heights is another advantage over fixed weirs. By determining the stage-discharge relationship at different angles, it can also be used as a flow measurement structure. The purpose of this study is to determine the stage-discharge relationship of the structure and its discharge coefficient at different openings.

The body of variable height whirling weir consists of two quarter circle sections on both sides and a rectangular section on the floor. At full opening, the rectangular section is placed horizontally and provides the maximum cross-sectional area for flow. By whirling the body, this structure acts like a weir and, while regulating the water level, also passes a specified discharge. A flume with a trapezoidal section with a length of 60.5 m was used to investigate the hydraulic behavior of VHW weir. The bottom width of this flume is 0.3 meters, the maximum depth is 0.25 meters, the side slope is 1:1 and the average slope is 0.0009. The VHW weir was installed at a distance of 44.5 meters from the beginning of the canal to create a uniform flow. To collect the required data, different weir openings were investigated in each specified discharge. Data including discharge, upstream water level of weir and angle of weir floor relative to the horizon were recorded. At each stage of the experiment, discharge was recorded by a flowmeter for two minutes and piezometer board was captured via digital photography. The recorded photos were digitized by Grapher software and the water depth in the all piezometers was determined. For determining the stage-discharge relationship of this structure in free flow condition, hydraulic, power and dimensional analysis methods were used.

In the hydraulic method, stage-discharge rating curves were plotted by the upstream water depth of the VHW weir and inlet discharge to the canal at different angles. Therefore, the discharge coefficient was determined for each opening. By obtaining the discharge coefficient for each opening, a relation can be written for the changes of the discharge coefficient versus the angle. Considering the relationship between the discharge coefficient and the angle, it can be seen that for angles larger than 35, the VHW weir had a different performance compared to smaller angles. the reason for changing the data trend can be attributed to increase the effect of the weir wall on the flow. In the power method by having the upstream water depth of the VHW weir and the inlet discharge to the canal, it is also possible to obtain a relation for the coefficient C and b versus the angle. In this method, the relationship trend changes at angle of 30 degrees. To generalize the results, the two dimensionless parameters which obtained from Buckingham theorem were plotted against each other. According to the graph and the data trend, the stage-discharge relationship can be divided into two parts. Data up to an angle of 35 degrees follows a trend, so it is best to use from one relation for angle of 7 to 35 degrees and another relation for angle of 35 to 50 degrees. Based on the statistical parameters, the obtained relationships based on dimensional analysis gave a better result.

discharge of VHW weir was obtained by three methods hydraulic, power and dimensional analysis. Comparison of the statistical parameters of these three methods shows that the relationship obtained from the dimensional analysis is most consistent with the data. The results show that the hydraulic behavior of the weir at angles larger than 35 degrees is different from smaller angles. The main reason for this difference is the effect of the structure body on the flow path.